This application involves non-invasive analysis of targets and relates to U.S. utility patent Ser. No. 11/048,694 filed on 31 Jan. 2005 titled “Frequency Resolved Imaging”, which is a continuation in part of U.S. utility patent application Ser. No. 11/025,698 filed on 29 Dec. 2004 titled “A Multiple Reference Analysis System”, the contents of both of which are incorporated herein by reference as if fully set forth herein. This application also relates to U.S. utility patent Ser. No. 11/254,965 filed on 19 Oct. 2005 titled “Correlation of concurrent non-invasively acquired signals” the contents of which are incorporated herein by reference as if fully set forth herein.
Non-invasive analysis is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process. In the case of quality control, it enables non-destructive imaging and analysis on a routine basis.
Optical coherence tomography (OCT), is a technology for non-invasive imaging and analysis. OCT typically uses a broadband optical source, such as a super-luminescent diode (SLD), to probe and analyze or image a target. It does so by applying probe radiation from the optical source to the target and interferometrically combining back scattered probe radiation from the target with reference radiation also derived from the optical source.
The typical OCT optical output beam has a broad bandwidth and short coherence length. The OCT technique involves splitting the output beam into a probe and reference beam, typically by means of a beam splitter, such as a pellicle, a beam splitter cube or a fiber coupler. The probe beam is applied to the system to be analyzed (the target). The reference beam is typically reflected back to the beam splitter by a mirror. Light scattered back from the target is combined with the reference beam in the beam splitter to form the measurement signal. Because of the short coherence length only light that is scattered from a depth within the target whose optical path length is substantially equal to the path length of the reference combine interferometrically. Thus the interferometric signal provides a measurement of the scattering value at a particular depth within the target. By varying the magnitude of the reference path length, a measurement of the scattering values at various depths can be determined and thus the scattering value as a function of depth can be determined, i.e. the target can be scanned.
In order to optimize the signal to noise ratio of the OCT imaging and analysis system the magnitude of the reference beam should be arranged to be compatible with the magnitude of the back scattered optical signal. This is typically achieved in conventional OCT systems by including a fixed attenuation element in the reference beam path. This technique is described in the paper titled “A Simple Intensity Noise Reduction Technique for Optical Low-Coherence Reflectometry” by authors W. V. Sorin and D. M. Baney published in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 4, No. 12, Pages 1404-1406, December 1992.
The magnitude of the fixed attenuator is typically selected to attenuate the reference beam that is combined with the back-scattered probe signal to a level that is not significantly higher than the maximum back scattered probe signal. However the magnitude of the optical signal back-scattered from within the target decreases rapidly (typically approximately logarithmically) with increasing depth. Therefore with the conventional fixed attenuation approach the reference beam has an increasingly higher level with respect to back-scattered signals from increasing depths. consequently the OCT signals associated with increasingly deeper regions within the target have increasingly reduced signal to noise ratio.
Furthermore in non-invasive imaging and analysis systems that use multiple reference beams, such as those described in application Ser. Nos. 11/025,698, 11/048,694, 11/254,965 incorporated herein by reference, there is typically a significant portion of the reference radiation reflected back to the beam splitter by the partial reflective element involved in generating the multiple reference beams. This component of the reference radiation that is reflected from the partial reflecting element introduces additional significant noise that degrades the signal to noise ratio.
Typically the magnitude of the reference radiation reflected from the partial reflecting element has a higher power level than the power levels of the multiple reference signals. Therefore introducing a fixed attenuator to reduce the magnitude of the radiation reflected from the partial reflecting element is not practical because it will attenuate the multiple reference signals by a corresponding amount.
There is therefore an unmet need for a method and apparatus for optimizing the level of the reference radiation or components of the reference radiation in order to enhance the signal to noise ratios and thereby improve system performance for both conventional OCT systems and multiple reference non-invasive analysis systems.